1. Field of the Invention
The present invention relates, in general, to refrigerating circuit apparatus. In particular, the present invention relates to a refrigerating circuit apparatus that is provided with an inverter circuit for controlling the rotation speed of a compressor.
2. Description of the related art
In a conventional air conditioning apparatus or display case, a refrigerating circuit typically includes a compressor, a condenser, a de-compressing device, e.g., expansion valve, and an evaporator. The refrigerating circuit also includes an inverter circuit supplying a driving frequency to the compressor. The output frequency of the inverter circuit is controlled in response to an air conditioning load or a refrigerating load to drive the compressor at a suitable ability, resulting in decrease in energy consumption of the compressor. The output frequency of the inverter circuit is controlled to minimize the difference (air conditioning load or refrigerating load) between the suction side refrigerant pressure Ps of the compressor and a set value Pc. In this case, a zone control method is employed to control the output frequency of the inverter circuit. A plurality of pressure deviation zones are defined in the zone control method. The output frequency of the inverter circuit is controlled such that changes of the output frequency of the inverter circuit are fast in a pressure deviation zone where the difference (pressure deviation) between the suction side refrigerant pressure Ps and the set value Pc is large. On the other hand, changes of the output frequency of the inverter circuit are slow in another pressure zone where the difference between the suction side refrigerant pressure Ps and the set value Pc is small.
In a practical manner, a refrigerating circuit apparatus includes a compressor, a condensor, an accumulator and a control circuit having an inverter circuit. Such a refrigerating circuit apparatus may be applied to different kinds of refrigerating apparatus, e.g., display cases, refrigerators, etc., having an evaporator and an expansion valve at a user side. Such refrigerating apparatus have different refrigerating loads, and thus, require suitable control operations that are different from one another. However, in the conventional refrigerating circuit apparatus, only one pattern of the changing speed of the output frequency fed from the inverter circuit is provided, although the changing speed of the output frequency in one zone is different from that in another zone. Thus, it is difficult to adapt the refrigerating circuit apparatus to different uses where each use or application requires a different control operation.
On the other hand, since a pseudo pulse width modulation (PWM) control method is generally used in the inverter circuit, the output waveform of the inverter circuit usually includes a lot of high frequency components as compared with a sine-wave component. Thus, vibration of the windings of the compressor motor may be caused by such high frequency components of the output waveform, and a vibration noise may occur from the motor. The tone of the vibration noise may change every time the output frequency of the inverter circuit changes. Such vibration noise becomes notably audible during nighttime. This is because the output frequency of the inverter circuit is changed in accordance with the conventional zone control method wherein only one pattern of the changing speed of the output frequency fed from the inverter is provided, even though the refrigerating load does not change frequently during nighttime.